A homogenous linearly- or circularly-polarized light field is easy to create using polarizing sheets, multilayer filters, crystal polarizers, and quarter-wave plates, as described for example by S. G. Lipson, H. Lipson, D. S. Tannhauser, “Optical Physics”, 3rd ed. Cambridge University Press (1995). It is also known how to produce a spatially-varying polarization field using liquid crystal devices as described by M. Stalder, M. Schadt, “Linearly polarized light with axial symmetry generated by liquid crystal polarization converters”, Optics Letters 21, 1948 (1996); diffractive optics as described by Z. Bomzen, G. Biener, V. Kleiner, E. Hasman, “Radially and azimuthally polarized beams generated by space-variant dielectric subwavelength gratings” Optics Letters, 27, 285 (2002); and holographic filters as described by P. B. Mumola et al, “Unstable resonators for annular gain volume lasers”, Applied Optics 17, 936-43, (1978).
However, the performance of these devices is usually dependent on the wavelength of the light used. Moreover, the latter two, diffractive optics and holographic filters, are relatively inefficient and are not applicable to visible light frequencies.
A device called a “reflexicon” is used as an element in annular laser resonators as described by Mumola and in R. A. Chodzko et al, “Annular resonators: some experimental studies including polarization effects”, Applied Optics 19, 778-89 (1980). However, although the reflexicon does indeed sometimes modify the polarization field in a non-homogeneous way, it is not designed for producing specific polarization fields and in its published form would not be able to do this, because its polarization characteristics are determined by its geometry and the Fresnel coefficients.
In short, it has not been known how to polarize visible light in a spatially variant manner efficiently and over a wide bandwidth.
The present invention produces a light beam with spatially varying polarization. In the present invention, a homogeneous beam of light is reflected from a first reflector onto a second reflector—one reflector being a concave (converging) reflector and the other reflector being a convex (diverging) reflector (the order of which may be reversed)—the second reflector returning it to a homogeneous beam, and a polarizer between the two reflectors is used to modify the polarization field of the output beam in the required manner. By “homogeneous” it is meant, for the purpose of the present invention a beam with random polarization.
The first reflector is used to reflect the initial beam so that at least some of it is reflected with a radial component. The radial component is polarized by a polarizing sheet, provided in a radial manner in between the first and second reflectors, so that the polarized light is then reflected from the second polarizer to the desired direction. For many purposes the light reflected off the second reflector would be directed parallel to the initial beam, but this is not necessary. By a combination of the shapes of the reflectors and of the polarizer, several useful polarization fields can be achieved.